Technical Abstract:
Both natural and managed ecosystems are currently exposed to an elevated carbon dioxide concentration ([CO2]) level that has not been experienced by terrestrial vegetation since the early Miocene and are facing a future that portends uncertain consequences of ever increasing [CO2]. Understanding how plants have and will respond to the rapid change in [CO2] along with developing knowledge about their capacity to adapt is an essential initial step in understanding the full impact that the multiple interacting factors of global change (e.g., drought, temperature, ozone) will have on terrestrial ecosystems. The performance of these ecosystems is critical to sustaining the security of ecosystem services on which the world depends for food, fuel, fiber, clean air and fresh water. An enormous amount has been learned about how plants respond to these projected future levels of [CO2] from various sorts of enclosure studies conducted over the past three decades. As scientific understanding advanced and underlying mechanisms were revealed, the need to test findings and hypotheses under truly open-air field conditions became increasingly apparent, leading to the development of a new technology - Free-Air Carbon dioxide Enrichment (FACE). In this review, we present six important lessons about impacts on plant carbon, nitrogen and water relations learned from FACE experiments and review the underlying evidence gleaned from these studies.